I didn't know it existed until I was looking for a planner solution today. I'm loving Google Calendar. You can even have reminders sent to your cell phone! (Of course, I have the cell phone company that it doesn't work with for some reason, but I found my own way around that.) You can create seperate calendars, but they all show up on one main calendar (that was kind of confusing for me at first), so it's great to sort.
My problem was that I had multiple calendars: one for work, one for personal, one to tell me when staff would be out, one for daily reminders. This takes care of it all in one! You can even download Google desktop so it'll work similar to Outlook's calendar.